The direct conversion of thermal energy into electrical energy (without the use of rotating machinery) is known in the art. This technology typically finds little practical application, however, as presently achievable conversion efficiencies are quite poor. For example, while such mechanisms as steam turbines are capable of conversion efficiencies in excess of about 50%, typical prior art direct conversion thermoelectric energy (TE) techniques offer only about 5 to 10% conversion efficiencies with even the best of techniques yielding no more than about 14% in this regard.
TE technologies generally seek to exploit the thermal energy of electrons and holes in a given TE material to facilitate the conversion of energy from heat to electricity. An expression to characterize the maximum efficiency for a TE power generator involves several terms including the important dimensionless figure of merit ZT. ZT is equal to the square of the Seebeck coefficient as multiplied by the electrical conductivity of the TE material and the absolute temperature, as then divided by the thermal conductivity of the TE material. With a ZT value of about 4, a corresponding TE device might be expected to exhibit a conversion efficiency approaching that of an ideal heat-based engine. Typical excellent state of the art TE materials (such as Bi2Te3-Bi2Se3 or Si—Ge alloys), however, have ZT values only near unity, thereby accounting at least in part for the relatively poor performance of such materials.
To reach a value such as 4 or higher, it appears to be necessary to maximize the power factor while simultaneously minimizing the thermal conductivity of the TE material (where the power factor can be represented as the product of the square of the Seebeck coefficient and the electrical conductivity). This has proven, however, a seemingly intractable challenge. This power factor and thermal conductivity are transport quantities that are determined, along with other factors, by the crystal and electronic structure of the TE material at issue. These properties are also impacted by the scattering and coupling of charge carriers with phonons. To maximize TE performance, these quantities seemingly need to be controlled separately from one another and this, unfortunately, has proven an extremely difficult challenge when working with conventional bulk materials.
Bulk refractory nanocrystalline powder materials are also known in the art and include, for example, disperse diamond powder. As bulk diamond comprises both an outstanding thermal conductor and a superb electrical insulator, bulk diamond is quite unsuited for use as a TE material for at least the reasons set forth above. In more recent times, however, nanocrystalline diamond material (having crystallite sizes of about 2 to 5 nanometers) has been successfully doped to achieve n or p-type electrically conducting material at ambient temperatures of interest while also exhibiting very low thermal conductivity. To date, however, no one has suggested a way to make good upon such possibilities and hopes for a useful TE material continue to remain mere unmet aspirations.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.